Evapotranspiration Estimates over Non-Homogeneous Mediterranean Land Cover by a Calibrated “Critical Resistance” Approach

Abstract

An approach based on the Penman-Monteith equation was used to estimate the actual evapotranspiration from local meteorological data over non-homogeneous land cover in a Mediterranean site in the south-east of Italy, with two six month data sets from two different years of measurements (2006 and 2009). The "critical resistance" formulation was used in different forms to model the surface resistance, together with some modifications to take into account the soil moisture content. One, two, or three model parameters were estimated, one of them related to the atmospheric resistance and the others to the surface resistance, and the calibration was made by either linear regression or nonlinear minimization of a proper cost function, depending on the applicability. Two kinds of cost functions were tested, the first depending on both the latent heat flux and the difference between screen air temperature and surface radiometric temperature, and the second depending on the temperature difference only. In all cases the calculated fluxes give better results with respect to both a flux-gradient approach and a complementarity based method, that require comparable data inputs. However the calibration by the temperature differences only, that requires no turbulent flux measurements, considerably increases the statistical uncertainty of the calibration parameters. The inclusion of the soil moisture did not significantly improve the model results in the considered site.

Land-Atmosphere Transfer Parameters in the Brazilian Pantanal during the Dry Season

“…A "hybrid" formulation of the PM model was proposed by Martano [14] in which a surface resistance can be expressed by the parallel between a soil surface resistance Rs and a canopy resistance Rv. This is supposed to be the simplest way to capture the main characteristics of partially-vegetated surfaces, with bare soil and vegetated surfaces giving independent contributions to the total evaporation, and is justified by the lack of detailed surface information.…”

“…that is Rv/Ra = f(R*/Ra), and used a linear approximation for the function f. Accordingly, the following expression is proposed for the vegetated fraction [14]:…”

“…An estimation of the uncertainty introduced by the blackbody approximation is given in the Appendix and is expected not to be greater than that associated with the difference between the radiometer field of view and the footprint area of the turbulent fluxes [14]. The uncertainty in the average values of Table 1 has been estimated as the standard deviation of the values obtained for all different couples of levels; although the estimated uncertainty by the M00 method for each value associated with a couple of a level is quite larger and of the order of the parameter value.…”

“…Instead, to obtain an estimate of the surface transfer coefficients, the approach outlined in Section 3.2 has been used together with the regression method described in [14], which will be shortly outlined here. To estimate the unknown parameters Ch, Cv and Cs for the simple model outlined in Section 3.2, a cost function F depending on both the latent heat flux and the surface temperature is written in the form [14]:…”

“…The undefined surface boundary condition prevents the application of this method to the water vapor profile. In this case, for the sake of completeness, the surface drag coefficient for the water vapor has been calculated applying a Penman-Monteith (PM) approach [1], with a procedure that needs a minimum requirement of surface information [14].…”